1. Field of Invention
The present invention relates to hardness testing, and more particularly to a hardness testing device comprising an electric coil coaxially and integrally wounded on a guiding tube so as to minimize a clearance between a test member and the electric coil for achieving more accurate hardness testing results.
2. Description of Related Arts
Hardness testing is extremely important for mechanical engineers. An accurate testing result effectively assists mechanical engineers in selecting appropriate materials and making an optimal engineering design in a wide variety of circumstances.
As a matter of conventional arts, there exists various hardness testing devices in which they can be broadly categorized as either handheld type or stationary type. One particular handheld type hardness testing device measures hardness of work pieces by Leeb Hardness Value wherein a test member, which is usually embodied as a predetermined type of magnetic materials, is driven to impinge against a work piece surface at a predetermined impact velocity. When the test member rebounds from the work piece surface, the impact velocity and a rebound velocity are recoded and interpreted according to a particular scientific formula and other engineering tolerances to come up with a Leeb Hardness Value.
Thus, a conventional hardness testing device of this type, as shown in FIG. 1 of the drawings, usually comprises a supporting tube 10P having an test opening 11P formed at a bottom thereof, a test member 20P moving along the supporting tube 10P, an actuation device provided on the supporting tube 10P and adapted to drive the test member 20P to impinge on the work piece surface so as to develop the impact velocity and the rebound velocity, and a hardness transducer 40P comprising a coil assembly 41P and a data processor. The coil assembly 41P is usually mounted on the supporting tube 10P at a position near the testing opening 11P and loaded with an electric current for producing a magnetic field inside the supporting tube at the portion surrounded by the coil assembly 41P. The data processor is electrically connected with the coil assembly 41P in such a manner that when the test member passes through the magnetic field at the impact velocity and the rebound velocity, the magnetic field would be interfered to an extent that the impact velocity and the rebound velocity can be determined by the data processor in accordance with conventional established scientific formulas. Moreover, once the impact velocity and the rebound velocity have been determined, the hardness of the work piece surface can also be determined as, for example, a Leeb Hardness Value (HL), by the following equation:HL=(Rebound velocity/Impact Velocity)×1000
A major drawback for this type of conventional hardness testing device is that the essence of the hardness testing depends on the impact velocity and the rebound velocity of the test member, which is driven by the actuation device. It follows that the manner of which the test member 20P hits the work piece surface is of crucial importance in developing the impact velocity and the rebound velocity. For example, the impinge angle of the test member directly affects the distance by which the test member 20P travels in the magnetic field so as to affect an accurate detection of the relevant velocities (the impact velocity and the rebound velocity). Therefore, it is expected that the greater the clearance between the test member 20P and the coil assembly 41P, the greater the error in detecting the relevant velocities. The reason is that according to simple electro-magnetic theory, the smaller the clearance between the test member 20P and the coil assembly 41P, the stronger the electric current induced in the coil 411P and therefore, the higher the measurement accuracy.
From this it is suggested that for the above-mentioned conventional hardness testing device, there usually exists a substantial clearance between the coil assembly 41P and the corresponding test member 20P. This clearance is expected to account for substantial measurements error during conventional hardness measurements. The reason for the existence of this substantial clearance is that the conventional coil assembly 41P usually comprises a holder 43P mounted to the supporting tube 10P while the coil 411P is wound on this holder. As a result, due to the physical dimension of this holder 43P, a substantial clearance between the coil 411P and the test member 20P is inevitably expected.